The grids are used for changing the angle of coverage of the light source. So, for example, if you use a 10 degree grid, then the light will cover about a 12" radius circle at 10' rather than probably covering the whole background without the grid. A 40 degree grid would cover a 48" radius circle at 10'. So in effect the grid gives you a spot light that controls the size of the circle of light that is projected on a scene at a given distance. You could use a grid to do any or all of the things you mention. But sometimes other modifiers might give more control. These might be snoots, or cinefoil, or louvres, or flags, or gobos, and so forth. Get a good book on lighting and you'll see examples of how these devices are used. Here is an example of using a light with a grid as a part of the scene itself:
Re: Give me the dirt on useing Grids
And here is a link to a nude shot that used to strip lights each with a grid and were also included in the scene.